This application is a 371 of PCT/EP00/01640, filed Feb. 28, 2000published Sep. 21, 2000.
The present invention relates to a process for the preparation of cyclic, a,p-unsaturated ketones by dehydrogenation of cyclic ketones at elevated temperature in the absence of oxygen or in the presence of less than 0.5 mol of oxygen per mol of starting ketone in the presence of catalysts having a surface area (BET) of at least 0.5 m2/g.
The preparation of cyclic xcex1,xcex2-unsaturated ketones from the corresponding saturated ketones in the vapor phase is described eg for cyclopentenone by F. Delles, J. Am. Chem. Soc. 91, 27 (1969) by uncatalyzed vapor-phase pyrolysis at from 532xc2x0 to 581xc2x0 C. in vacuo. However, the cyclopenenone yield and selectivities that can be obtained are very low, since degradation reactions predominate.
U.S. Pat. No. 3,364,264 further describes the conversion of ketones to enones in the vapor phase over oxidation catalysts in the presence of at least one mol of oxygen per mol of ketone. The examples do not reveal what yields or selectivities are achieved, as there are no statements on the overall conversion or by-products. Apart from the actual yields which may be achieved, this process suffers from the basic drawback that when it is dimensioned for large-scale operation special safety measures are necessary, since metering of the large amounts of oxygen is carried out near to or within the explosive limits.
It is thus an object of the present invention to provide a process by means of which cyclic xcex1,xcex2-unsaturated ketones can be continuously synthesized in an industrially simple manner in the vapor phase in high yields without any risk of uncontrolled decomposition.
This object is achieved in the present invention by a process for the preparation, in particular the continuous preparation, of cyclic, xcex1,62 -unsaturated ketones of formula II by dehydrogenation of cyclic ketones which conform to formula I 
in which n denotes an integer from 1 to 10, and which may be substituted, at elevated temperature in the presence of catalysts in the vapor phase, where the reaction is carried out in the absence of oxygen or in the presence of less than 0.5 mol of oxygen per mol of compound I at temperatures ranging from 250xc2x0 to 600xc2x0 C. and using catalysts having a surface area (BET) of more than 0.5 m2/g.
The cyclic enones and ketones of formulas II and I respectively can carry substituents on the cyclic carbon atoms if desired. Such substituents are eg alkyl or aryl radicals. Preferred starting ketones are cyclohexanone and, in particular, cyclopentanone.
The reaction is preferably carried out in the absence of oxygen, but the reaction may, if desired, be carried out in the presence of specific amounts of oxygen, eg up to 0.2 mol of oxygen per mol of ketone.
The reaction temperatures used in the process of the invention are generally between 2500 and 600xc2x0 C. and preferably range from 300xc2x0 to 575xc2x0 C. and particularly from 350xc2x0 to 550xc2x0 C. The reaction pressure is generally between 0.1 and 10 bar and preferably ranges from 0.5 to 6 bar and more preferably from 0.9 to 3 bar.
The conversion of the cyclic ketones can be carried out with or without a carrier gas. If a carrier gas is used, inert gases such as methane, nitrogen and argon are suitable.
If oxygen is to be metered in, this is effected either by metering in air or mixtures of air with inert gas or metering in pure oxygen or mixtures of oxygen with inert gases. During this operation the molar ratio of oxygen to ketone should not usually exceed 0.1:1. Preferably the molar ratio is below 0.01:1 and more preferably the process is carried out in the absence of oxygen.
The process of the invention can be carried out in the presence of water. The amount of water used is generally not critical, but it is preferably between 0.01 and 1 kg and more preferably between 0.05 and 0.2 kg of water per kg of cyclic ketone. The addition of water generally causes prolongation of the catalyst onstream time.
Suitable catalysts are theoretically all solids which undergo no change under the conditions of the reaction and have a surface area (BET) greater than 0.5 m2/g. Preferred catalysts have surface areas greater than 1 m2/g. There is theoretically no upper limit to the surface area that can be used, but for practical considerations the surface area (BET) will not usually exceed 2000 m2/g.
Accordingly, the surface area (BET) of the catalysts to be used in the present invention generally ranges from 0.5 to 2000, preferably from 1 to 500 and more preferably from 2 to 200 m2/g.
Particularly suitable catalyst compositions are oxidic materials. These primarily comprise catalysts containing or consisting of oxides of Group 2 to Group 14 elements.
Examples thereof are magnesium oxide, titanium(IV) oxide, zirconium(IV) oxide, chromium(III) oxide, cobalt oxide, copper oxide, zinc oxide, alumina and silicon dioxide.
The oxides can be uniform or mixed. Different oxides can form a homogeneous mixed oxide when precipitated together or they can exist in the form of a mechanically produced blend.
Other examples of catalytically active base materials having a large surface area are activated carbon and silicon nitride.
The aforementioned catalytic materials having a large surface area may contain, eg in amounts of from 0.01 to 5 wt %, compounds of Group 1, Group 15 and Group 16 elements. These elements are eg Na, K, P or S, usually in the form of their oxides.
Finally, metallic components, preferably likewise in the form of the oxides of Group 7 to Group 11 elements, may be aditionally applied, particularly in the case of said oxidic base materials. As examples thereof there may be mentioned rhenium, ruthenium, cobalt, rhodium, nickel, palladium, platinum, copper and silver.
Specifically, the following catalysts are suitable for example:
CuO, AgO, PdO, NiO, Mn2O3 or Re2O7 
on ZnO, CaO, BaO, SiO2 or Al2O3.
The preparation of these catalysts is carried out in known manner, for example by impregnation and calcination or by precipitation of hydroxides or carbonates followed by calcination, as described, for example, in Charles N. Satterfield, Heterogeneous Catalysis in Industrial Practice, 2nd Edition, New York, McGraw Hill.
The process of the invention can be carried out over fixed or agitated catalysts. Examples of suitable reactor types are fixe-dbed reactors, fluidized-bed reactors and riser reactors.
When the catalyst activity falls, for example due to organic deposits, the catalyst, if of inorganic nature, can generally be fully regenerated by burning-off the sedimentation in air, eg at from 400xc2x0 to 500xc2x0 C.
The conversion can be carried out in a straight pass, the resulting mixture of products then being purified and any unconverted educt recycled, or the reaction product can be recycled to the reactor prior to purification in order to increase the conversion rate.
Cyclic enones such as cyclopentenone or cyclohexenone are sought-after intermediates for the preparation of pharmaceutical agents or agricultural pesticides.